A gas giant is a massive planet made mostly of hydrogen and helium, with no solid surface to stand on. Jupiter and Saturn are the two gas giants in our solar system, and thousands more have been discovered orbiting other stars. Instead of rock and dirt beneath your feet, these worlds have swirling layers of gas that gradually thicken into liquid the deeper you go, surrounding a relatively small dense core.
What Gas Giants Are Made Of
The defining feature of a gas giant is its composition. Jupiter and Saturn are overwhelmingly hydrogen and helium, the same elements that make up stars. Jupiter is about 90% hydrogen and 10% helium by volume. Saturn has a similar mix. Trace amounts of other compounds like ammonia, methane, and water vapor create the colorful cloud bands visible from space, but these are minor ingredients in an enormous ball of light gases.
This is fundamentally different from rocky planets like Earth, Mars, or Venus. Those worlds have thin atmospheres draped over large solid bodies. Gas giants flip that relationship: they have small, dense cores buried under massive layers of gas and liquid. The core of a gas giant is thought to be a mix of rock and metal, but it accounts for only a fraction of the planet’s total mass.
Layers Without Boundaries
One of the strangest things about gas giants is that there’s no clear line between the atmosphere and the “body” of the planet. On Earth, you can point to where the ground starts. On Jupiter, the hydrogen atmosphere simply gets denser and hotter as you descend, gradually shifting from gas to liquid without any sharp transition. There’s no surface, no ocean edge, no moment where you’d say “I’ve landed.”
Go deeper still, and the pressure becomes so extreme that hydrogen enters an exotic state called liquid metallic hydrogen. At this point, the hydrogen behaves like a metal, conducting electricity. On Jupiter, this transition happens at pressures around 3 million times Earth’s sea-level atmospheric pressure. This electrically conductive layer is what generates Jupiter’s powerful magnetic field. Saturn has a similar metallic hydrogen layer, though it sits deeper inside the planet.
At the very center, Jupiter’s core reaches temperatures estimated above 20,000°C, with pressures that may hit 100 million times what you feel at sea level on Earth. NASA’s Juno spacecraft, which has been orbiting Jupiter since 2016, revealed that the core isn’t a neat, compact ball as scientists expected. Instead, Jupiter has a “dilute” or “fuzzy” core, where heavy elements are mixed into the surrounding hydrogen rather than sitting in a tidy clump. That finding has reshaped how scientists think Jupiter formed.
How Gas Giants Form
Gas giants start out the same way rocky planets do: small bits of rock and ice clump together in the disk of material swirling around a young star. The key difference is what happens once that rocky core reaches a critical size, roughly 5 to 20 times the mass of Earth. At that point, the core’s gravity becomes strong enough to pull in huge quantities of hydrogen and helium gas from the surrounding disk.
This triggers a runaway process. As gas piles on, the planet’s gravity strengthens, which pulls in even more gas, which strengthens gravity further. In a relatively short window of time (by cosmic standards), the planet balloons from a rocky seed into a gas-dominated giant. This has to happen while the star’s gas disk is still present, typically within the first few million years of a solar system’s life. If the disk dissipates before the core gets big enough, you end up with something smaller, like an ice giant.
Gas Giants vs. Ice Giants
Uranus and Neptune are sometimes lumped in with Jupiter and Saturn, but planetary scientists draw a clear distinction. Jupiter and Saturn are gas giants, made primarily of hydrogen and helium. Uranus and Neptune are ice giants, made primarily of hydrogen compounds: water, ammonia, and methane. These “ices” aren’t frozen solid inside the planet. They exist as superheated, pressurized fluids. But because their composition is so different from Jupiter and Saturn, the two categories are kept separate.
All four of these outer planets share some traits. They rotate much faster than rocky worlds (Jupiter completes a full rotation in just under 10 hours), and that rapid spin flings material outward at the equator, giving them a slightly flattened, oblong shape rather than a perfect sphere. They all have ring systems, though Saturn’s are by far the most spectacular. And they all have cores made of some combination of rock, metal, and hydrogen compounds.
Size and Scale
Jupiter is about 11 times wider than Earth. If Earth were the size of a grape, Jupiter would be roughly the size of a basketball. Saturn is about 9 times wider than Earth, only slightly smaller in diameter than Jupiter, though significantly less massive because it’s less dense. Saturn is actually less dense than water. If you could find a bathtub big enough, it would float.
Despite their enormous size, gas giants are dwarfed by their host stars. Jupiter contains about one-thousandth the mass of the Sun. It would need to be roughly 80 times more massive to ignite hydrogen fusion and become even the faintest type of star.
Weather on a Giant Scale
Without a solid surface to slow things down, gas giant atmospheres produce staggering wind speeds. Jupiter’s equatorial jet stream blows at around 300 kilometers per hour, comparable to jet streams in Earth’s upper atmosphere. Saturn’s equatorial winds reach 1,800 kilometers per hour. Neptune, though technically an ice giant, holds the record at 2,100 kilometers per hour.
Jupiter’s Great Red Spot is the most famous storm in the solar system. It’s a counterclockwise-rotating storm that has persisted for more than 300 years, since at least the invention of the telescope. When the Voyager spacecraft flew past in 1979, the storm measured about 25,000 kilometers long. By the end of the Galileo mission in 2000, it had shrunk to around 20,000 kilometers, and it continues to shrink today, though it’s still wider than Earth. On a planet with no solid surface to create friction and break up weather systems, storms this large can last centuries. Smaller storms on Jupiter persist for decades.
Neptune once had its own version: a Great Dark Spot nearly 10,000 kilometers long, spotted by Voyager 2 in 1989. But when the Hubble Space Telescope looked for it in the mid-1990s, the storm had vanished entirely.
Gas Giants Around Other Stars
Gas giants were among the first types of exoplanets discovered, largely because their size makes them easier to detect. The most dramatic category is the “hot Jupiter,” a gas giant that orbits extremely close to its parent star. Nearly 200 known exoplanets have masses larger than half of Jupiter’s and complete an orbit in less than four days. Some circle their star in as little as 18 hours. For comparison, Mercury takes 88 days to orbit the Sun.
Hot Jupiters were a surprise. Nothing like them exists in our solar system, where gas giants orbit far from the Sun in cold, outer regions. Their existence forced scientists to rethink planetary migration, the idea that planets don’t necessarily stay where they form but can spiral inward or outward over time. These scorching worlds have surface temperatures of thousands of degrees, with atmospheres unlike anything seen on Jupiter or Saturn.
Why We Can’t Visit the Surface
There is no surface to visit, and even sending a probe deep into the atmosphere is extraordinarily difficult. NASA’s Galileo mission dropped a probe into Jupiter’s atmosphere in 1995. It transmitted data for about 58 minutes before the increasing pressure and temperature destroyed it, and it only penetrated a tiny fraction of the way toward the core.
The challenge is straightforward: pressure and temperature increase relentlessly with depth. Jupiter’s core pressure may reach 100 million times Earth’s surface pressure, at temperatures exceeding 20,000°C. No known material can survive those conditions. When fragments of Comet Shoemaker-Levy 9 slammed into Jupiter in 1994, a single fragment released energy equivalent to 6 million megatons of TNT and created a fireball peaking at 40,000°C, with a plume rising more than 3,000 kilometers. Gas giants are beautiful from a distance, but they are violently inhospitable up close.